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F. Emran, J. Rihel, A. R. Adolph, J. E. Dowling; Larval Zebrafish Lose Vision at Night. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3054.
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Studies in several species have shown that a number of retinal phenomena are regulated by circadian mechanisms. For example, synaptic ribbons in photoreceptor terminals in larval zebrafish undergo dramatic diurnal alterations. The ribbons are very prominent during the day, but are almost completely absent at night. To determine whether these structural alterations in photoreceptor terminals relate to visual responsiveness in larval zebrafish, we recorded electroretinograms (ERGs) as well as carrying out two behavioral tests, the optokinetic reflex (OKR) and visual motor response (VMR), at different times of the day and night.
ERGs were recorded from 5dpf larval zebrafish at different times of the day and night using the isolated eye preparation (Wong et al., Zebrafish, 2004). The OKR and VMR behavioral tests (Emran et al., PNAS, 2007) were used to assess the visual responsiveness of larval zebrafish at 5dpf.
We observed obvious differences in outer retinal function in day versus night. ERG responses in larval zebrafish are normal in terms of waveform and amplitude throughout the day, but the ERG is almost completely absent after several hours of darkness at night. When larval fish are maintained under normal lighting conditions during the night, they exhibit smaller but otherwise normal ERGs. The results suggest a circadian cycle component alters the responsiveness of the outer retina at night, but this circadian component can be over-ridden by exposure to light. The OKR and VMR behavioral assays confirm the ERG recordings, showing that visual responsiveness is severely diminished at night. OKR tests performed on fish exposed to light during the night show that visual responsiveness partially recovers, again in agreement with the ERG results.
ERG recordings and behavioral testing show that larval zebrafish become virtually unresponsive to light stimuli at night. Light exposure of larvae at night restores visual responsiveness partially, indicating that the loss of responsiveness is due to both circadian and other mechanisms.
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